A variety of lasers are used in a variety of applications for converting electrical energy into optical energy. For example, in optical communications networks, various types of lasers are used to convert electrical data signals into optical data signals, which are then transmitted over optical waveguides, e.g., optical fibers. One type of laser that is sometimes used for this purpose is an EML, which is a combination of a distributed feedback (DFB) laser diode and an electro-absorption modulator (EAM) integrated on a single optoelectronic integrated circuit (IC) chip. An EML driver IC chip that is external to the EML chip delivers electrical signals to the EAM to cause the EAM to modulate the DFB laser diode.
FIG. 1 is a schematic diagram of a typical EML driver IC chip 2 connected to an impedance matching network 3. For ease of illustration, the EML chip is not shown in FIG. 1. The impedance matching network 3 has a contact pad 4 that connects to an input contact pad of an EML IC chip (not shown). The EML driver IC chip 2 has driver electronics 5 that drive the bases of a differential pair of bipolar junction transistors (BJTs) 6 and 7. The emitters of the BJTs 6 and 7 are connected to one another. A current source 8 represents the output current, IO, of the differential pair 6, 7. The collectors of the BJTs 6 and 7 are connected to respective load resistors (RL1 and RL2) 11 and 12 that are connected in parallel, where RL1=RL2=50 Ohms (Ω) to provide an output impedance of 25Ω. The load resistors RL1 11 and RL2 12 are connected to a supply voltage, VDD. The driver electronics 3 drive the bases of the BJTs 6 and 7 to push the output current IO into the 25Ω output impedance provided by the parallel load resistors RL1 11 and RL2 12, which causes an output voltage to be produced at the output terminals 13 and 14 of the chip 2 that swings between a positive output voltage, VOUT_P, and a negative output voltage, VOUT_N.
The EML driver IC chip 2 and the EML chip operate at different supply voltages. The impedance matching network 3 matches the output impedance of the EML driver IC chip 2 to the input impedance with the EML chip (not shown). The impedance matching network 3 shown in FIG. 1 generally has become the standard configuration used for EML chips. In order for the impedance matching network 3 to provide the impedance needed to match the output impedance of the EML driver IC chip 2 with the input impedance of the EML chip (not shown), a pair of transmission lines 16 and 17 are used and the EML driver IC chip 2 is located a relatively large distance away from the EML chip. The transmission line 16 has a first portion 16a and a second portion 16b that are interconnected by an alternating current (ac) capacitor 18, which decouples the common modes of the EML driver IC chip 2 from the common modes of the EML chip. The impedance matching network 3 also includes inductors L1 21, L2 22 and L3 23, termination resistor RT 25, and termination capacitor CT 26. Contact pad 27 is for connecting the EML chip to the impedance matching network 3 to provide the bias current to the EML chip. Contact pad 4 is for providing the modulation current to the EML chip. A termination network 29 is included to replicate the circuit configuration made up of components 18, 21, 22, 23, 25, and 26.
One of the disadvantages of the arrangement shown in FIG. 1 is that the impedance of the transmission lines 16 and 17 reduces the output voltage swing of VOUT_P and VOUT_N by about 50%. Because of the reduction in the output voltage swing, IO must be increased to achieve the desired output voltage swing, which leads to higher power consumption. Another disadvantage of the arrangement is that the transmission lines 16 and 17 prevent the EML driver IC chip 2 from being placed in close proximity to the EML chip, which makes it difficult to decrease the overall footprint of the layout that includes the EML driver IC chip 2, the impedance matching network 3 and the EML chip (not shown). Yet another disadvantage of the arrangement shown in FIG. 1 is that the termination network 29 increases the complexity and cost of the impedance matching network 3.
A need exists for a low-power, direct-drive EML driver circuit that eliminates the need for the transmission lines 16 and 17 and the termination network 29, thereby allowing power consumption and costs to be reduced without reducing bandwidth, and allowing the EML driver circuit to be placed in close proximity to the EML chip.